Add items to an existing book
An existing note can be added to a book by clicking on the note you want to add, then selecting Entry menu > Add current item to book...
The status bar will change to indicate that you are in Add to Book mode:
Now just click the book you want to add the note to, and it will be added to the book.
Adding search resuls to a book
You can select Entry menu > Add all search results to book... after performing a search. The search result will be saved internally, then cleared and you will be prompted to click the book to add these notes to, as above.